Final answer:
When humans exercise, they burn up energy that was stored in their muscles as glycogen, which is a form of complex carbohydrates.
Step-by-step explanation:
When humans exercise, they burn up energy that was stored in their muscles as complex carbohydrates in the form of glycogen. Glycogen is a polysaccharide, made up of many glucose molecules linked together, and it serves as a quick source of energy during physical activity. As humans engage in exercise, glycogen is broken down into glucose, which is then used for energy production.
Learn more about energy storage in muscles during exercise